In recent years, a growing interest in renewable energy has led to the widespread use of solar panels, which receives sunlight and generates electric power, to ordinary homes and ordinary businesses. Mega solar power plants wherein solar panels are disposed in a relatively large space are also built in many places. However, when a solar panel is placed on the ground, in many cases, the area under the solar panel is not utilized. Taking such a problem into consideration, an invention described in the following [Patent Literature 1] discloses a technique wherein an agricultural plant is cultivated on the ground under a solar panel.